Arrangement comprising at least two optical elements and a plasma generating device

Abstract

Arrangement comprising at least two optical elements (14, 15), in particular mirror elements for semiconductor technology, and a plasma generating device (30, 40, 50, 60, 70) The optical elements (14, 15) each have an optical surface (16, 17), wherein the optical surfaces (16, 17) are aligned with one another in such a way that a cavity (20) is formed between the optical surfaces (16, 17) and an incoming beam path is incident directly successively on the optical surfaces (16, 17) in order thus to be converted into an outgoing beam path. The plasma generating device (30, 40, 50, 60, 70) is positioned in the region of an optical surface (16, 17) of at least one of the optical elements (14, 15) and / or in the region of an opening (18, 28) leading to the cavity (20) in order to generate a plasma suitable for cleaning the optical surfaces (16, 17) within the cavity. The invention allows plasma cleaning within the cavity without the optical elements having to be demounted.

Description

[0001] 07 . 01 .2026 / BS

[0002] Arrangement comprising at least two optical elements and a plasma generating device

[0003]

[0001] This patent application claims the priority of German patent application DE 10 2025 100 815.9, filed on January 12, 2025, the entire disclosure of which is hereby incorporated by reference . The invention relates to an arrangement having at least two optical elements, in particular mirror elements for semiconductor lithography, and a plasma generating device . The invention furthermore relates to an optical system comprising a radiation source for generating a beam path and comprising an arrangement according to the invention, and to a method for plasma treatment of optical surfaces of at least two optical elements .

[0004]

[0002] Optical elements are used in semiconductor technology to shape light generated by a radiation source and convert it into a beam path suitable for the respective application purpose . In order to ensure the imaging quality of the optical elements, it is customary to clean the optical surfaces regularly. In the case of easily accessible optical surfaces, this can be done with the aid of a plasma treatment, by means of which for example organic contaminants or other layers that adversely affect the optical properties, in particular the reflectivity, can be effectively removed. However, optical surfaces situated opposite one another in such a way that a cavity is formed between them are accessible only with difficulty for prior art plasma cleaning methods . In the prior art, it is therefore customary firstly to remove the optical elements from the optical system for the purpose of carrying out the cleaning and then to subj ect them to a plasma cleaning. However, the requisite work for demounting the optical elements and the necessary readjustment of the optical elements after cleaning are time-consuming and costly. What is particularlycomplex is the treatment of cavity-forming optical elements whose optical surfaces oxidize easily on account of their constitution and therefore cannot be removed from a vacuum environment .

[0005]

[0003] Against this background, the obj ect of the present invention is to provide an arrangement, an optical system and a method which allow simpler cleaning of optical surfaces which form a cavity together . This obj ect is achieved by the features of the independent claims . Advantageous embodiments are specified in the dependent claims .

[0006]

[0004] Accordingly, the invention relates to an arrangement comprising at least two optical elements, in particular mirror elements for semiconductor technology, and a plasma generating device . The optical elements each have an optical surface, wherein the optical surfaces are aligned with one another in such a way that a cavity is formed between the optical surfaces and an incoming beam path is incident directly successively on the optical surfaces in order thus to be converted into an outgoing beam path. A cavity is formed between the optical surfaces, wherein the plasma generating means is positioned in the region of an optical surface of at least one of the optical elements and / or in the region of an opening leading to the cavity in order to generate a plasma suitable for cleaning the optical surfaces within the cavity.

[0007]

[0005] In the context of the invention, it has been recognized that positioning the plasma generating device in the region of the optical surface and / or in the region of an opening leading to the cavity makes it possible to generate a homogeneous plasma within the cavity, the plasma enabling a uniform, thorough and residue-free cleaning of the optical surface . Arranging the plasma generating device in direct proximity to the optical elements makes it possible to introduce energy dir-ectly into the cavity or into an opening leading to the cavity with the aid of the plasma generating device . In this way, a gas situated in the cavity or a gas situated in direct proximity to the cavity can be put into the plasma state . The plasma cleaning of the optical surfaces can be performed in this way without necessitating removal of the optical elements from the arrangement in order to expose the optical surfaces . Rather, the optical elements can remain in their installation state, in which they are aligned in such a way that the incoming beam path is incident directly successively (i . e . without any other optical elements in between) on the optical surfaces and is thus converted into an outgoing beam path.

[0008]

[0006] The arrangement can be positioned in a closed chamber, in particular in a vacuum chamber, in which a gas is present which can be put into the plasma state by the operation of the plasma generating device . The chamber can also be part of the arrangement . The arrangement can furthermore comprise a gas supply device designed to supply a gas to an effect region of the plasma generating device and / or to set a defined gas environment within the vacuum chamber .

[0009]

[0007] The cavity has an opening. The opening can be formed by an interspace between the optical surfaces that is present in the edge region of the opposing optical surfaces . The interspace forming the opening can extend over the entire circumference of the opposing optical surfaces, i . e . can be formed for example by a ring gap . An opening leading to the cavity can also be positioned centrally in one of the optical elements . It is also possible for the opening to be situated in the region of an optical axis of the optical element . An opening situated in the region of the optical axis can be designed in particular as a passage for the beam path. The optical elements can be obscured mirrors in particular .

[0008] The optical elements can have diffractive optical elements and reflective optical elements (hereinafter also referred to as mirror elements) . A mirror element can have in particular a mirror body formed from a substrate and provided with a reflective coating (mirror surface) , which forms the optical surface . The cavity can be formed in particular by two mirror elements positioned opposite one another, the mirror surfaces of which are aligned with one another in such a way that the incoming beam path firstly is incident on a first of the mirror surfaces and is directed from this to the second of the mirror surfaces .

[0010]

[0009] A distance between the opposing optical surfaces can be between 0.5 cm and 30 cm, preferably between 1 cm and 20 cm. The distance can be greater in the centre than in an edge region of the opposing optical surfaces . For example, the distance in an edge region of the optical surfaces can be between 0.5 cm and 5 cm. Furthermore, the distance in the centre of the optical surfaces can be between 3 cm and 15 cm. A volume of the cavity can be between 100 and 6000 cm3, in particular between 200 and 3000 cm3.

[0011]

[0010] The plasma generating device can have in particular an induction coil designed to put a gas situated in an effect region of the induction coil into a plasma state by inductive coupling. This can be done in a fundamentally known manner by applying a radio-frequency AC voltage, so that the gas is put into the plasma state by inductive coupling. In particular, at least one winding of the induction coil can enclose the cavity, so that the effect region extends to the interior of the cavity. If the windings of the induction coil enclose the cavity, i . e . the cavity is at least partially situated within the induction coil, the plasma generating device can act directly on the interior of the cavity and generate a plasma there . It can be provided that the induction coil surrounds a ring gapformed between the optical elements . Furthermore, a winding axis of the induction coil can be aligned substantially parallel to an optical axis of at least one of the optical elements . If the induction coil surrounds the cavity as symmetrically as possible, a uniform plasma can be generated within the cavity, and has a homogeneous cleaning effect on the optical surfaces . In the present case, the alignment between two axes is considered to be "substantially parallel" if an angular deviation between the axes is less than 15° , preferably less than 10° , further preferably less than 5° .

[0012]

[0011] The plasma generating device can have a tube portion leading into the cavity via at least one of the openings . At least one winding of the induction coil can enclose the tube portion, so that the effect region of the induction coil extends to an interior of the tube portion. Furthermore, the arrangement can be designed for generating a pressure gradient, by which a plasma generated in the tube portion is conveyable into the cavity. The tube portion can have in principle any cross-sectional shape, wherein the cross-sectional shape can be circular in particular . Within the tube portion, a plasma can be generated by inductive coupling and can be conveyed into the cavity with the aid of the pressure gradient . A suitable setting of the pressure gradient allows the optical surfaces to be uniformly impinged on by the plasma and effectively cleaned in this way.

[0013]

[0012] If at least one of the optical elements has a mirror body having a mirror surface forming the optical surface, the induction coil can be at least partially integrated into the mirror body. The space requirement can be reduced as a result . By virtue of the induction coil being integrated into the mirror body, i . e . in particular embedded in the material of the mirror body, the induction coil can additionally be positioned at a very small distance from the cavity, whereby the induct-ive coupling to the gas situated in the cavity is intensified. However, it is also possible in principle for the induction coil to act on a gas volume situated outside the cavity and for a plasma generated outside the cavity subsequently to be guided into the cavity.

[0014]

[0013] In one embodiment, the plasma generating device is designed for generating EUV radiation. In this case, the plasma generating device can be aligned in such a way that the EUV radiation is radiated via the opening into the cavity in order to put gas situated in the cavity into a plasma state . In this embodiment, the EUV radiation radiated into the cavity results in an ionization of the gas molecules and an energy transfer between the EUV radiation and the electrons released in the cavity, as a result of which the gas is put into the plasma state . As a result, a homogeneous plasma can be ignited along the direction of irradiation, and acts uniformly on the optical surfaces .

[0015]

[0014] In one embodiment, the plasma generating device is designed for generating an electron beam and is aligned in such a way that the electron beam is radiated via the opening into the cavity in order to put gas situated in the cavity into a plasma state . In this case, collisions between the electrons radiated in and the electrons of the gas molecules situated in the cavity result in an ionization of the gas molecules and an energy transfer from the electrons radiated in to the released electrons, as a result of which the gas can be put into the plasma state . In this case, too, a homogeneous plasma can be generated along the direction of irradiation, and uniformly cleans the optical surfaces .

[0016]

[0015] According to a further embodiment, the plasma generating device is designed for generating microwaves and is aligned in such a way that the microwaves are radiated via theopening into the cavity in order to put gas situated in the cavity into a plasma state . The microwave radiation enables energy to be transferred to the electrons of the gas molecules situated in the cavity and an ionization of the gas molecules to be achieved. It is possible for the plasma generating device to be designed to radiate microwaves into the cavity via a plurality of openings or from a plurality of directions, wherein it is furthermore possible for a threshold required for ignition of the plasma to be exceeded by a superposition of the microwaves within the cavity. As a result of the ionization of the gas molecules, a high free electron density can arise locally in the cavity, and can lead to a high absorption of the microwaves, which can be detrimental to the homogeneity of the plasma . Therefore, provision can furthermore be made for microwaves of different frequencies to be radiated into the cavity. By using microwaves of different frequencies, it is possible to modify the spatial distribution of the generated electrons and thus of the critical electron density in such a way that the plasma is situated homogeneously in the centre of the cavity.

[0017]

[0016] The arrangement can furthermore comprise a plurality of magnetic elements tuned to the plasma generating device in such a way that charge carriers situated in the cavity are excited to a cyclotron resonance by the microwaves . Magnetic fields can be generated within the cavity by means of the magnetic elements, and free electrons which can be generated in particular by ionization of the gas molecules in the cavity are forced on circular paths by said magnetic fields if microwaves with a suitable frequency are radiated in. In this way, the electrons can absorb the energy of the microwaves particularly efficiently. The generation of a plasma with the aid of electron cyclotron resonance is known in principle in the prior art, and so a detailed explanation can be omitted at this juncture .

[0017] The magnetic elements can be positioned uniformly around a circumferential surface of the cavity. If the optical element has a mirror body having a mirror surface, the magnetic elements can be integrated into the mirror body. This allows the magnetic elements to be positioned very close to the cavity, so that a strong magnetic field can be generated in the region of the optical surfaces within the cavity. This makes it possible to generate the plasma in a targeted manner in the vicinity the optical surfaces, so that the plasma can act on the optical surfaces in a targeted manner and produces a homogeneous cleaning effect .

[0018]

[0018] In one embodiment, the plasma generating device is electrically connected to at least one of the optical surfaces in order to put a gas situated in the cavity into a plasma state by capacitive coupling. In this case, the optical surface can be a metallic mirror surface in particular . The optical surface can comprise one or more metal layers applied to a mirror body. The optical surface can also be the surface of a metallic mirror body. In this case, the plasma generating device can be designed in particular for generating a radiofrequency electrical AC voltage . The AC voltage can be applied to one of the optical surfaces, while the other of the optical surfaces can be connected to an earth potential . It is also possible for an antiphase electrical voltage to be applied to the optical surfaces by the plasma generating device . The generation of a capacitively coupled plasma is known in principle in the prior art and therefore does not need to be explained in greater detail in the present case . Since the mirror surfaces regularly extend over a large part of the cavity, a particularly homogeneous plasma filling the entire cavity can be generated in this embodiment .

[0019]

[0019] The invention furthermore relates to an optical system comprising a radiation source for generating a beam path, com-prising an arrangement according to the invention and comprising a sensor device for detecting an image representation of the beam path. The beam path on the way from the radiation source to the sensor device as an incoming beam path is incident firstly on a first of the optical surfaces and then on a second of the optical surfaces and is thus converted into the outgoing beam path. The optical system can have further optical elements in addition to the first and second optical elements . The disclosure encompasses further embodiments of the optical system, which is developed further by features described in connection with the arrangement according to the invention. Moreover, the disclosure encompasses further embodiments of the arrangement according to the invention, which are developed further by features of the optical system according to the invention. The first and second optical elements of the optical system can be subj ected to a plasma treatment in a simple manner, with removal of the optical elements from the optical system not being necessary for this purpose .

[0020]

[0020] The invention furthermore relates to a method for plasma treatment of optical surfaces of at least two optical elements which are arranged opposite one another in such a way that a cavity is formed between the optical surfaces, comprising the following steps :

[0021] - positioning a plasma generating device in a region of an optical surface of at least one of the optical elements and / or in the region of an opening leading to the cavity;

[0022] introducing a gas into an effect region of the plasma generating device;operating the plasma generating device in such a way that gas situated in the effect region transitions into a plasma state .

[0023]

[0021] The disclosure encompasses further embodiments of the method, which are developed further by features described in connection with the arrangement according to the invention. Moreover, the disclosure encompasses further embodiments of the arrangement according to the invention, which are developed further by features of the method according to the invention .

[0024]

[0022] The invention is explained by way of example below on the basis of exemplary embodiments with reference to the accompanying drawings, in which:

[0025] Figure 1 : shows a first embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device;

[0026] Figure 2 : shows a second embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device;

[0027] Figure 3 : shows a third embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device;

[0028] Figure 4 : shows a fourth embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device;

[0029] Figure 5 : shows a fifth embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device;Figure 6 : shows a sixth embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device;

[0030] Figure 7 : shows a seventh embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device

[0031] Figure 8 : shows an eighth embodiment of an arrangement according to the invention comprising two optical elements and a plasma generating device;

[0032] Figure 9 : shows a schematic illustration of an optical system according to the invention.

[0033]

[0023] Figure 1 shows a schematic sectional illustration of a first embodiment of an arrangement according to the invention. The arrangement comprises a first optical element 14 and a second optical element 15, which are positioned opposite one another in such a way that a cavity 20 is formed between the optical elements 14, 15.

[0034]

[0024] The optical elements 14, 15 each comprise an optical surface 16, 17. In the present case, the optical elements 14, 15 are designed as mirror elements and each comprise a mirror body 19, to which a reflective coating forming the respective optical surface 16, 17 is applied. In an alternative embodiment, the mirror body 19 can also be a metallic mirror body, wherein the optical surface is a surface of the metallic mirror body. In a sectional plane oriented perpendicularly to the drawing plane, the optical elements 14, 15 have a circular cross section. The optical surfaces 16, 17 are concavely curved .

[0035]

[0025] The optical elements 14, 15 are arranged at a distance from one another which is about 1 cm in an edge region andsignificantly larger in a central region and can be up to 10 cm. On account of the distance, a ring-shaped opening 18 is present in an edge region of the cavity, through which opening a connection is established between an environment of the optical elements 14, 15 and the cavity 20. Each of the mirror bodies 19 additionally comprises an opening 28 in a central region, which opening extends along an optical axis of the optical elements 14, 15. The optical elements are thus designed as obscured mirrors .

[0036]

[0026] A first of the openings 28 is used to admit an incoming beam path, not illustrated in the present case, into the cavity 20. The beam path firstly is incident on the mirror surface 17 and, after reflection at the latter, is relayed directly to the mirror surface 16. After reflection at the mirror surface 16, the beam path can emerge from the cavity 20 again as an outgoing beam path through the other of the openings 28.

[0037]

[0027] A plasma generating device 30 is situated in the region of the ring-shaped opening 18. The plasma generating device 30 comprises an induction coil 31, the windings of which extend in a circumferential direction circularly around the ringshaped opening 18. This causes the cavity to be enclosed by the induction coil 31 or by the windings thereof . The plasma generating unit 30 is connected to a control unit 24, which is configured to apply a radio-frequency AC voltage to the induction coil 31 .

[0038]

[0028] The optical elements 14, 15 and the induction coil 31 are arranged in a vacuum chamber 22. A pump device 23 is connected to the vacuum chamber 22, said pump device being configured for evacuating the vacuum chamber 22 and for feeding a defined amount of gas into the vacuum chamber 22. The gas can be hydrogen or argon, for example .

[0029] In order to carry out the method according to the invention, in the vacuum chamber 22 firstly a defined amount of gas is introduced into the vacuum chamber 22 or a defined gas pressure is produced with the aid of the pump device 23. The gas can enter the cavity 20 via the openings 18 and 28. Subsequently, a radio-frequency AC voltage is applied to the induction coil 31 with the aid of the control unit 24 in order to generate an inductively coupled plasma within the cavity 20. The physical processes taking place here are known in principle and therefore do not need to be explained in detail in the present case . By virtue of the induction coil 31 being arranged substantially symmetrically around the cavity 20, a homogeneous plasma can be generated within the cavity, and enables the optical surfaces 16, 17 to be uniformly cleaned.

[0039]

[0030] Figure 2 shows a schematic sectional illustration of a second embodiment of an arrangement according to the invention. The elements that have already been described in connection with Figure 1 bear the same reference signs in Figure 2. Only the differences between the embodiment in Figure 1 and the embodiment in Figure 2 are described below.

[0040]

[0031] In the embodiment in Figure 2, the plasma generating device 30 is arranged in the region of the opening 28 and is embedded in the mirror body 19 of the optical element 14. In this embodiment, the induction coil 31 of the plasma generating device 30 surrounds the opening 28 to the cavity 20. The induction coil 31 is arranged substantially symmetrically with respect to an optical axis of the optical element 14. In this way, a plasma can be ignited in a central region of the cavity 20 in proximity to the optical axis, and can spread homogeneously within the cavity and thus cause uniform cleaning.

[0041]

[0032] Figure 3 shows a schematic sectional illustration of a third embodiment of an arrangement according to the invention.Such elements which are present identically or analogously in the embodiment in Figure 1 bear the same reference signs in Figure 3. Only the differences between the embodiments in Figure 1 and Figure 3 are described below.

[0042]

[0033] In contrast to the embodiment in Figure 1, the plasma generating device 30 in Figure 3 comprises a tube portion 32 inserted into the opening 18 and leading into the cavity 20. In addition, the induction coil 31 of the plasma generating device 30 is embedded in the mirror bodies 19 of the two optical elements . The windings of the induction coil 31 are guided around the outside of the tube portion 32, so that the tube portion is enclosed by the windings . A longitudinal axis of the tube portion 32 is aligned substantially parallel to a central axis defined by the windings . Furthermore, in the present case, the pump device is connected to an inlet of the tube portion 32. The pump device 23 is configured to supply the inlet of the tube portion with a predefined gas volume flow. In an alternative embodiment, not illustrated here, the tube portion can also protrude outwardly from the cavity, wherein the windings of the induction coil are guided around the outside of the tube portion, and wherein the induction coil is positioned next to the optical elements and is not embedded in the optical elements .

[0043]

[0034] A connection 25 is situated on an opposite side of the ring-shaped opening 18 with respect to the tube portion 32, a further conveying device 26 being connected to said connection. The conveying device 26 is designed for generating a reduced pressure .

[0044]

[0035] In order to carry out the method according to the invention, a vacuum or a defined low gas pressure is firstly set in the vacuum chamber 22 with the aid of the pump device 23. Subsequently, the pump device 23 supplies the tube portion 32with a defined gas volume flow and the control unit 24 applies a radio-frequency AC voltage to the induction coil 31, so that the gas introduced into the tube portion 32 is put into a plasma state . In addition, the conveying device 26 is put into operation, so that a reduced pressure is generated via the connection 25 on the opposite side of the cavity 20 with respect to the tube portion 32 . The plasma generated in the tube portion 32 is conveyed through the cavity 20 in this way. On the way through the cavity 20, the plasma can uniformly have a cleaning effect on the optical surfaces 16, 17.

[0045]

[0036] Figure 4 shows a schematic sectional illustration of a fourth embodiment of an arrangement according to the invention. Such elements which are present identically or analogously in the embodiments in Figures 1 to 3 bear the same reference signs in Figure 4. Only the differences between the embodiments in Figures 1 to 3 and the embodiment in Figure 4 are described below.

[0046]

[0037] In the embodiment in Figure 4, the plasma generating device, unlike the previous embodiments, does not comprise an induction coil . Rather, the plasma generating device 40 is configured for emitting EUV radiation 41 in the present case . In the present case, the plasma generating device 40 is positioned adj acent to the opening 18 leading to the cavity 20 in such a way that the EUV radiation passes centrally through the cavity 20 and emerges again from the cavity 20 through the opening 18 on the opposite side .

[0047]

[0038] The high-energy EUV radiation ionizes the gas molecules situated in the cavity on its way through the cavity, so that said gas molecules are put into the plasma state . Along the direction of irradiation predefined by the EUV radiation 41, a homogeneous plasma can be generated within the cavity 20 inthis way, and leads to a uniform cleaning of the optical surfaces 16, 17 .

[0048]

[0039] Figure 5 shows a schematic sectional illustration of a fifth embodiment of an arrangement according to the invention. Such elements which are present identically or analogously in the embodiments in Figures 1 to 4 bear the same reference signs in Figure 5. Only the differences between the embodiment in Figure 4 and the embodiment in Figure 5 are described below .

[0049]

[0040] In the embodiment in Figure 5, the plasma generating device 50 is configured for emitting an electron beam 51. The plasma generating device 50 is positioned adj acent to the opening 18 leading to the cavity 20 in such a way that the electron beam 51 passes centrally through the cavity 20 and emerges again from the cavity 20 through the opening 18 on the opposite side . The plasma generating device 50 is connected to the control unit 24 and obtains control commands from the latter .

[0050]

[0041] The high-energy electron beam ionizes the gas molecules situated in the cavity 20 on its way through the cavity, so that the gas is put into the plasma state . Along the direction of irradiation predefined by the electron beam 51, a homogeneous plasma can be generated within the cavity 20 in this way, and leads to a uniform cleaning of the optical surfaces 16, 17 .

[0051]

[0042] Figure 6 shows a schematic sectional illustration of a sixth embodiment of an arrangement according to the invention. Such elements which are present identically or analogously in the embodiments in Figures 1 to 5 bear the same reference signs in Figure 6. Only the differences between the embodimentin Figure 5 and the embodiment in Figure 6 are described below .

[0052]

[0043] In the embodiment in Figure 6, the plasma generating device 60 is configured for emitting microwaves 62. For this purpose, the plasma generating device 60 comprises two microwave sources 61, which are positioned on opposite sides of the cavity 20 respectively adj acent to the opening 18 leading to the cavity 20 in such a way that the microwaves 62 enter the cavity 20 from opposite sides and are superposed within the cavity 20. The microwave sources 61 are connected to the control unit 24 and obtain control commands from the latter .

[0053]

[0044] The microwaves 62 transfer energy to the electrons situated within the cavity 20. With suitable selection of the microwave power, microwave frequency and by means of a suitable arrangement and alignment of the microwave sources, a state can be achieved in which a plasma is ignited on account of the superposition of the microwaves 62 in the centre of the cavity 20. The plasma ignited in the centre spreads homogeneously within the cavity 20 and leads to a uniform cleaning of the optical surfaces 16, 17.

[0054]

[0045] Figure 7 shows a schematic sectional illustration of a seventh embodiment of an arrangement according to the invention. Such elements which are present identically or analogously in the embodiments in Figures 1 to 6 bear the same reference signs in Figure 7. Only the differences between the embodiment in Figure 6 and the embodiment in Figure 7 are described below. The embodiment in Figure 7, just like the embodiment in Figure 6, comprises a plasma generating device 60 having two microwave sources 61, which introduce microwaves into the cavity 20 from opposite sides thereof . In addition, the embodiment in Figure 7 comprises a plurality of magnetic elements 63, which are integrated into the mirror bodies 19 ofthe optical elements 14, 15 and are situated at a small distance from the respective optical surface 16, 17. The magnetic elements 63 can be formed by permanent magnets or else by electromagnets . A distance between a magnetic element 63 and the respective optical surface 16, 17 can be in the range of between 0.1 mm and 5 cm, in particular between 0.2 cm and 2 cm.

[0055]

[0046] By means of the magnetic elements 63, a magnetic field, in particular a static magnetic field, can be generated within the cavity 20, and electrons moving within the cavity 20 can be accelerated in a circular path by said magnetic field. In the present case, the magnetic field generated by the magnetic elements 63 is tuned to the radiated-in microwaves in such a way that the electrons are excited to an electron cyclotron resonance . In this way, the electrons can absorb the energy introduced by the microwaves particularly effectively. The magnetic field elements 63 can be positioned in particular uniformly around the cavity 20, so that an area within which a constant field strength prevails is generated as uniformly as possible within the cavity 20. In particular, the area of the same magnetic field strength can be at a uniform distance from the optical surfaces 16, 17. In this case, the microwaves can be tuned to the magnetic field in such a way that an ignition of the plasma takes place in the region of the areas of the same magnetic field strength. This enables a particularly homogeneous plasma to be ignited along the optical surfaces, which has a very uniform cleaning effect . It is also possible for different magnetic elements 63 to generate regions with different magnetic field strengths within the cavity 20, wherein the microwaves radiated into the respective regions each have a frequency adapted to the different magnetic field strengths .

[0047] Figure 8 shows a schematic sectional illustration of an eighth embodiment of an arrangement according to the invention. Such elements which are present identically or analogously in the embodiments in Figures 1 to 7 bear the same reference signs in Figure 8. In contrast to the previous embodiments, the embodiment in Figure 8 has a plasma generating device 70 comprising a first contact element 71 and a second contact element 72. The first contact element 71 is electrically connected to the mirror surface 16 and the second contact element 72 is electrically connected to the mirror surface 17. The contact elements 71, 72 are embedded in the mirror body 19 of the respective optical element 14, 15.

[0056]

[0048] In the present case, the mirror surfaces 16, 17 are formed by a plurality of metal layers and have a high electrical conductivity. The two contact elements 71, 72 are connected to the control unit 24. In the present case, the control unit 24 is designed to apply a radio-frequency AC voltage to the contact element 71. The contact element 72 is connected to a constant potential (earth potential) . As a result of the radio-frequency AC voltage, the gas situated within the cavity 20 can be put into a plasma state by capacitive coupling. The generation of a plasma by capacitive coupling is known in principle in the prior art and therefore does not need to be explained in detail in the present case . By virtue of the mirror surfaces 16, 17 surrounding the cavity almost completely, in this embodiment a particularly homogeneous plasma can be generated, and can achieve a uniform cleaning effect .

[0057]

[0049] Figure 9 shows a schematic view of an optical system according to the invention, in which an arrangement 100 according to the invention is used. The optical system comprises a radiation source 81 for emitting a beam path 82. The beam path 82 can be shaped by optical elements 83, which are merely schematically indicated in Figure 9, before it is successivelyincident on the optical elements of the arrangement 100 according to the invention as an incoming beam path 84, in order to then be relayed as an outgoing beam path 85. In the beam path downstream of the arrangement according to the invention, further optical elements 86, which are merely schematically indicated in Figure 9, can be present, via which the beam path is guided to an image plane 87. By way of example, a sensor device designed for detecting a measurement signal can be positioned in the region of the image plane 87. In the present case, the optical system is arranged within a vacuum chamber 88, to which a pump device 89 is connected. Therefore, in the present case, the arrangement 100 has neither its own vacuum chamber nor its own pump device . The pump device 89 is designed to perform the functions of the pump device 23 described in connection with the embodiments in Figures 1 to 8 . If, after a specific operating time of the optical system, cleaning of the optical elements of arrangement 100 becomes necessary, this cleaning can be carried out in a simple manner with the aid of the method described in association with Figures 1-8, without necessitating removal of the optical elements 14, 15 from the arrangement 100.

Claims

Patent Claims1. Arrangement comprising at least two optical elements ( 14, 15) , in particular mirror elements for semiconductor technology, and a plasma generating device (30, 40, 50, 60, 70) , wherein the optical elements ( 14, 15) each have an optical surface ( 16, 17 ) , wherein the optical surfaces ( 16, 17 ) are aligned with one another in such a way that a cavity (20) is formed between the optical surfaces ( 16, 17 ) and an incoming beam path is incident directly successively on the optical surfaces ( 16, 17 ) in order thus to be converted into an outgoing beam path, wherein the plasma generating device (30, 40, 50, 60, 70) is positioned in the region of an optical surface ( 16, 17 ) of at least one of the optical elements ( 14, 15) and / or in the region of an opening ( 18, 28 ) leading to the cavity (20) in order to generate a plasma suitable for cleaning the optical surfaces ( 16, 17 ) within the cavity.

2. Arrangement according to Claim 1, wherein the plasma generating device has an induction coil (31 ) designed to put a gas situated in an effect region of the induction coil (31 ) into a plasma state by inductive coupling.

3. Arrangement according to Claim 2, wherein a winding of the induction coil (31 ) encloses the cavity (20) , so that the effect region extends to the interior of the cavity (20) , wherein the winding is aligned preferably substantially symmetrically with respect to an optical axis of at least one of the optical elements ( 14, 15) .

4. Arrangement according to Claim 2 or 3, wherein the plasma generating device (30) has a tube portion (32 ) leading into the cavity via at least one of the openings ( 18 ) , wherein a winding of the induction coil (31 ) encloses the tube por-tion (32 ) , so that the effect region of the induction coil (31 ) extends to an interior of the tube portion (32 ) , wherein the arrangement is furthermore designed for generating a pressure gradient by which a plasma generated in the tube portion (32 ) is conveyable into the cavity (20) .

5. Arrangement according to any of Claims 2 to 4, wherein at least one of the optical elements ( 14, 15) comprises a mirror body ( 19) having a mirror surface forming the optical surface ( 16, 17 ) , wherein the induction coil (31 ) is at least partially integrated into the mirror body ( 19) .

6. Arrangement according to any of Claims 1 to 5, wherein the plasma generating device (40) is designed for generating EUV radiation (41 ) and is aligned in such a way that EUV radiation (41 ) is radiated via the opening ( 18 ) into the cavity (20) in order to put gas situated in the cavity into a plasma state .

7. Arrangement according to any of Claims 1 to 6, wherein the plasma generating device (50) is designed for generating an electron beam (51 ) and is aligned in such a way that the electron beam (51 ) is radiated via the opening ( 18 ) into the cavity (20) in order to put gas situated in the cavity (20) into a plasma state .

8. Arrangement according to any of Claims 1 to 7, wherein the plasma generating device ( 60) is designed for generating microwaves ( 62 ) and is aligned in such a way that the microwaves ( 62 ) are radiated via the opening ( 18 ) into the cavity (20) in order to put gas situated in the cavity (20) into a plasma state .

9. Arrangement according to Claim 8, which furthermore has a plurality of magnetic elements ( 63) tuned to the microwaves( 62 ) in such a way that charge carriers situated in the cavity (20) are excited to a cyclotron resonance by the microwaves ( 62 ) .

10. Arrangement according to any of Claims 1 to 9, wherein the plasma generating device (70) is electrically connected to at least one of the optical surfaces ( 16, 17 ) in order to put a gas situated in the cavity (20) into a plasma state by capacitive coupling.

11. Arrangement according to any of Claims 1 to 10, which furthermore comprises a vacuum chamber (22 ) , in which the optical elements ( 14, 15) and the plasma generating device (30, 40, 50, 60, 70) are positioned.

12. Arrangement according to any of Claims 1 to 11, which furthermore has a gas supply device (23) , which is designed for introducing a gas into an effect region of the plasma generating device (30, 40, 50, 60, 70) .

13. Arrangement according to any of Claims 1 to 12, wherein the cavity has a volume of between 100 and 10, 000 cm3.

14. Optical system comprising a radiation source ( 81 ) for generating a beam path ( 82 ) , an arrangement according to any of Claims 1 to 13 and a sensor device ( 87 ) for detecting the beam path ( 82 ) , wherein the beam path on the way from the radiation source to the sensor device ( 87 ) as an incoming beam path ( 84 ) is incident on the optical surfaces ( 16, 17 ) and is converted into the outgoing beam path ( 85) by the optical surfaces ( 16, 17 ) .

15. Method for plasma treatment of optical surfaces of at least two optical elements ( 14, 15) which are arranged opposite one another in such a way that a cavity (20) is formedbetween the optical surfaces ( 16, 17 ) , comprising the following steps :- positioning a plasma generating device in a region of an optical surface ( 16, 17 ) of at least one of the op- tical elements ( 14, 15) and / or in the region of an opening leading to the cavity (20) ;- introducing a gas into an effect region of the plasma generating device;- operating the plasma generating device in such a way that gas situated in the effect region transitions into a plasma state .

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